Rotary peening apparatuses of the type illustrated in FIG. 1 have broad application in the abrasives field, for such things as cleaning, descaling, or otherwise abrading concrete, metal or other surfaces. This can be done to put a desired finish or texture on the surface, or to prepare the surface for a subsequent operation such as welding, painting or coating. As used herein, "high-intensity peening" is meant to include peening operations such as stress-relieving of metals, surface conditioning operations such as cleaning and descaling (i.e. removal of oxide scales and/or paint) of metals, creation of anti-slip surfaces, and surface conditioning of concrete. The peening apparatus includes a cylindrical hub having a plurality of flap slots, each of which retains one end of a peening flap. Each of the peening flaps includes one or more peening particle support bases positioned for contact with a workpiece surface when the hub is rotated about its central axis. The peening particle support bases each include on their exposed faces a plurality of hardened peening particles. These faces successively contact the workpiece surface during rotation of the hub, thereby abrading the workpiece surface.
One type of known peening flap and fastener is shown in FIGS. 2-6A. The peening flap 10 includes a strap 16, to which is fastened a plurality of peening particle support bases 24 each having peening particles 26 provided on the exposed face of each support base. Shank 25 extends to the interior of flap 10, and washer 23 is placed over the end of the shank 25 before the shank is flared to secure it to the flap. It is known to use a polymeric washer 23 to help withstand the impact of peening and to minimize cracking of the washer that was occurring with metal washers 23. Such peening flaps are available from Minnesota Mining and Manufacturing Company, of St. Paul, MN., and are known commercially as Heavy Duty Roto Peen flaps, Type B, Type C, and Type D. Peening flap 10 has a peening end 14 and a hub end 12. Strap 16 includes a first end 18, second end 22, and a medial portion 20 where a fastener 50 secures together overlapping portions of the ends to the medial portion. As seen in FIG. 5, flap 10 can optionally include wear pad 40 and support strap fin 39. This general construction of the peening flap 10 is further described in several sources, including U.S. Pat. No. 5,203,189. The construction of the peening particle support bases described above is further described in U.S. Pat. No. 5,179,852. The contents of both of these patents are incorporated herein by reference.
Known fastener 50 is shown in greater detail in FIG. 6, and includes rivet body 51 and metal washer 64. Rivet 51 includes a head 52 atop fastener shaft 60. Head 52 includes convex head top surface 66 and planar head bottom surface 68 which meet at joining edge 70. Shaft 60 adjoins head bottom surface 68 centrally thereof, and passes through aligned apertures 28 formed in each respective portion of strap 16. Peening flap 10 and strap 16 may have many different configurations, and could, for example, include more or less overlying segments of strap 16 than the three that are shown in FIG. 3 or the four that are shown in FIG. 5. The distal end of the shaft 60 projects from flap 10 sufficiently to enable a cooperative fastener member 64 to be secured by flaring fold down stem 62 to form rollover flare 62'. Known peening flap fasteners employ a steel back up washer as cooperative fastener member 64.
The peening flap having known fastener 50, although having its own utility, can display certain disadvantages. Among them is a phenomenon referred to as "premature failure" of the peening flaps, which is defined as imminent or actual separation of the strap portions from the fastener before the useful life of the peening support base and particles is exhausted.
It is desirable to impart a sufficient amount of compression to the fastened portions of strap 16 when assembling flap 10 with fastener 50. This reduces the slippage of the fastened portions of the strap relative to one another and relative to fastener 50 during use of peening flap 10. Such slippage can lead to increased heat and stress, reducing the life of the strap 16. Compression by the fastener 50 also helps keep abraded particles from becoming lodged between the strap 16 and the fastener 50, and between the fastened portions of the strap. Such debris can also reduce the useful life of strap 16. However, too much compression can cause edge 70 of the fastener head to tear into or otherwise damage strap 16.
In an attempt to minimize damage to the fabric of strap 16 by known fastener 50, while at the same time providing a desired amount of compression to the portions of the strap 16 fastened by fastener 50, assembly typically is as follows. Fastener shaft 60 is passed through apertures 28 in the strap 16, steel washer 64 is then placed over the end of shaft 60, and the fold down stems 62 of the fastener rivet 51 is mechanically deformed or flared to maintain the washer 64 on the shaft and to set a predetermined distance between head 52 and washer 64. This distance is determined empirically as described in greater detail below, and imparts the desired amount of compression to the fastened portions of strap 16.
Intense mechanical action occurs during high intensity peening. This includes centrifugal loads on the peening flap 10 as it is rotated at high speed about the hub, the impact loads caused by the peening particle supports bases 24 impacting the surface to be abraded, and bending of the strap 16 caused by the peening impact. These mechanical actions cause frictional heat and stress in the strap portions in the area of the fastener 50, which is increased by the heat and stress caused by the slippage of the fastened portions of the strap relative to one another and relative to the fastener 50. Additionally, head 52 and metal washer 64 tend to dig into and otherwise damage strap 16. All of this tends to cause the fastened strap portions to become thinner, thereby reducing or eliminating the amount of compression initially imparted by fastener 50 when assembling peening flap 10.
Because there is insufficient elastic deformation in known fastener 50, especially in metal washer 64, the initial compressive load is quickly lost as the fastened portions of strap 16 become thinner from wear, stretching, compression, or other causes. This increases the slippage of the fastened strap portions relative to one another and relative to fastener 50, thereby further increasing the heat and stress during operation and increasing the likelihood of premature strap failure. Loss of compression also allows severely abrasive abraded debris to become lodged between the fastener and the strap and between the overlapped portions of the strap. This, too intensifies the heat and stress during high intensity peening and increases the likelihood of premature failure. Lodged debris becomes more of a problem as rotary peening devices are more frequently used to clean or scarify more brittle surfaces such as concrete. The sharp intersection 70 of rivet head top surface 66 and bottom surface 68 also contributes to premature failure. Furthermore, recent improvements in the materials and methods used to fabricate strap 16 and peening particle supports 24, and improved designs for peening flap 10, have extended the useful life of these components. See for example, U.S. Pat. Nos. 5,179,852, 5,203,189, and 5,298,303. As a result, the premature failure caused by fastener 50 has become the life limiting factor in using peening flaps 10.
A peening flap 10 exhibiting premature failure is shown in FIG. 7, wherein strap 16 has begun to tear away from fastener heads 52 at the hub side 29 of apertures 28. Peening particle support bases 24 and particles 26, although partially worn, still have useful life and are suitable for abrading a surface. Because a portion of strap 16 could completely detach from fasteners 50 with further use of the peening flap, a peening flap in such a condition is typically discarded even though the peening particles 26 and support bases 24 have not yet exhausted their useful lives. Therefore, this system results in unnecessary waste and increased costs, as the entire peening flap 10 must be replaced prior to exhausting the useful life of all of the components.
In view of the disadvantages of conventional peening flaps and fasteners, an unmet need exists for a durable fastener for a peening flap that will secure the end portions of the strap to the medial portion and that will withstand harsh operating conditions for a sufficient time that the full useful life of the peening heads can be enjoyed.